1. Field of the Invention
The present invention relates to a laser velocimetric probe and, more particularly, to a probe which includes means for generating interference fringes and measurement means for determining the velocity of a particle passing through the fringe field from a signal representing the light backscattered by this particle.
2. Discussion of Related Art
Velocimetric probes of this already known type generally consist of the following elements:
- a collimated coherent light source, such as a diode laser, delivering a light beam at a wavelength .lambda.; PA1 - an optical splitter giving, from the beam emitted by the source, two mutually parallel identical beams separated by a distance a; PA1 - an optical collimation device which focuses the previous beams at a distance F from its exit face. It may be demonstrated that, in the focusing zone, the two beams interfere. The interference field consists of straight, parallel fringes, the fringe spacing of which is given by: EQU i=.lambda.F/a PA1 - an optical reception device which forms the image of the focusing zone on a photoreceiver; PA1 - this photoreceiver and its control electronics; PA1 - an electronic processing unit giving an item of velocity information from the information coming from the photoreceiver.
Such a probe operates in the following manner.
When a particle passes through the fringe field, it backscatters the light toward the optical reception device and the photoreceiver. This light signal is modulated by the fringe field. It may be demonstrated that the temporal modulation frequency f is given by EQU f=v.sub.x /i
where v.sub.x is the particle velocity in the direction perpendicular to the fringes.
The electronic processing unit enables the frequency f, and therefore the velocity v.sub.x, to be obtained, for example by the Fourier transform method.
Such a probe enables just one component of the particle velocity to be determined. When it is desired to know two or three components, so as to determine the velocity and the incidence of the particle with respect to the probe, two or three optical heads are used, the fringe fields of which are mutually perpendicular. Such an arrangement has the two drawbacks of taking up a lot of space and of doubling or tripling the cost of the system.